Electric Go-Kart Made From Off The Shelf Components

What would you do with a catalog of parts and a nice budget? [Ben Rothschild] decided to build an electric go-kart from scratch, for a contest he’s trying to win.

He designed the entire go-kart in 3D CAD using off the shelf components to speed up assembly. The frame is made of aluminum extrusion with t-bolt brackets, and he’s using modified FIRST Robotics wheels with standard #25 chain and sprockets.

Two 1850W Turnigy SK3 brushless motors make up the drive system, equivalent to almost a 5HP engine — except with a constant torque profile, meaning it’ll have no problem going up hills at 3km/h or 30km/h, no gearing necessary! To power the beast he’s using four hard-shell LiPo batteries (4S1P), which are rated for 14.8V and 5Ah. Two el cheap-o 24V 500W speed controllers (slightly concerning) provide the control system, which he may plan to upgrade in the near future.

The test drive video is a bit short, but it looks like with a bit more work this go-kart could have a lot of potential!

Hi there, Not all of us can buy a Go Pro for one video – I was using an old Flip Video camera strapped to the front of the go cart while someone else filmed me with an iPhone. Tell you what, though, go to the Misumi Facebook link in the above post and help me win the contest and then if I win I will try and use it to buy a Go Pro and use that for the rest of my projects.

To put out 5 hp (3.7 kilowatts of power) at 15 volts would require 250 amps. I can’t find any specs but I doubt that any 5 amp hour lithium battery can put out more than a couple amps even at peaks. Even with lead acid batteries, capacitors, or something else to fill in the surges there’s no way that makes any sense.

https://sites.google.com/site/tjinguytech/charging-how-tos/the-variable-c
The variable C
Even though C is printed in some way on every lipo we buy, many people do not fully understand what it is or how to figure it out. Simply put C is a variable just like X from your algebra days. It is tied to a capacity of a lipo and as such can be used to figure things like charge rates and maximum continuous discharge rates.

Since C is tied to the capacity of the pack, it can be used to estimate charge times, discharge times and stress. For example if a pack is discharged at 1C, it will be totally discharged in 1 hour. It will also be charged in 1 hour when fully discharged. At 2C those time are cut roughly in half and at 3C those times will be cut down to roughly 1/3rd.

Figuring out what C is for a particular pack is simple, just take the capacity of the pack in mAh, divide it by 1000 to get Ah and then drop the “h”, that is C.

For example C for a 2200mAh 3s pack is 2.2A. Start by dividing 2200mAh by 1000 to get 2.2Ah. Then drop the “h” to get C=2.2A. It is as simple as that but here are a few other examples

The batteries are rated for 20C constant discharge and 30C discharge for 20 seconds max. That gives us 100 amps continuous per battery.

He has wired the lipos to make two 8S2P packs, one for each motor. This means he’s got 200 amps continuous that he is able to supply to each of the motors. If we to the math with the 30C 20 second discharge we get 150A per lipo = 300A per pack per motor.

This leaves us with a total of 600A that he can dump into motors for a maximum of 20 seconds. 600A at 15V gives us 9000W…

@John
Don’t your current estimates just mean that he’s got beefier motors than he can drive or is it more complicated than that. I would like to think that you can under-run a motor without losing much efficiency or getting other problems.
If you really have to go into detailed design calculations to get a working system it’s going to cause havoc with my plans to try to make something similar – I never studied electrics, just electronics.

“except with a constant torque profile, meaning it’ll have no problem going up hills at 3km/h or 30km/h, no gearing necessary!”

The whole point of gears is to turn speed into torque, so you could get better acceleration. You aren’t going to go 120 mph on a 5 HP motor anyways, so you might as well make it accelerate faster, so saying that you don’t need gearing is missing the point entirely or just not understanding motors.

Ok, someone might want to quietly and rationally explain to me what is so “special” about using incredibly and unbelievably expensive (and not forgetting hard to get) aluminium extrusion for such a simple frame?

You are right about it being expensive, except it is not hard to get at all. search “80/20” – which is the “made in the usa” super expensive stuff. They sell a lot of their parts that fail their very strict visual quality requirments on the ‘bay. Like if the anodizing comes out too bright for example. There are also plenty of metric knock offs on there for this stuff as well. I just built a complex “machine” out of that stuff, all bought on that auction site. But yeah, the U.S. made stuff is crazy expensive. I would have used the metric stuff if I did it all over again. I also found that it’s a lot heavier than I had imagined it would be. I’d suggest looking for the profiles you need in the “lite or ultralite” version unless you have super heavy duty needs.

Can’t agree with you on the price and availability though, I live in the country that produces the most aluminium in the world (excluding China who has just recently taken the mantel) – but it’s (for some reason that escapes me) more economical to export it, so we locals pay 2-4 times the price that you do – IF it is available…

We used to have general outlets with scrap bins and all, but now if you don’t have an order for a house full of window frames forget it..

(Oh and the greenies have forced electricity prices up through the roof and you know what that means to aluminium production…)

“There are seven alumina refineries operating in Australia producing mostly smelter grade alumina for both the domestic and export markets. Australia is the world’s second largest producer and exporter of alumina, with 22 per cent of global production. In 2011 Australia produced 19.1 million tonnes of metallurgical (smelter grade) alumina and around 0.5Mt of chemical grade alumina.”

I suppose it is safe to say that we are now more an exporter of raw materials now than a manufacturer of finished products as manufacturing has become uneconomical.

We have extremely cheap power via coal power stations but the Labor(sic)/Green coalition introduced a carbon dioxide tax and unreasonable so called renewable energy targets that have seen the cost of electricity sky rocket in the last 7 years – on top of that the power infrastructure had been allowed to run down so now there is a levy to fix that up as well. The irony is that we now export most of our coal to China so it still ends up in the atmosphere – absolute madness.

This is a toy, not a go-kart. The whole frame construction is a joke, the wheels are a joke, the motors are under-powered and you won’t be able to do any decent karting with this toy. Heck, even a traditional iron frame with an engine from a chainsaw is better.
Well, at least you tried… Let’s see if you find the motivation for version 2.0.

I will also leave my usual reminder here in light of some of the commentary. Remember that there are many avenues of getting into building as a hobby, and all of them are ultimately contributory to the space of design projects and builds people can learn from.

In part the inspiration for creating Chibikart and other similar designs it was derived from and which it has spawned (like this one) is to encourage use of these industrial and commercial components. These designs are purposefully more closely tied to industrial supplier catalogs, CAD programs, and modern rapid manufacturing techniques that a new engineering student might come to learn in school. It is less ‘heavy metal’ than welding a steel tube frame and using a lawn mower engine or similar, but it is just another way to approach the design in a realm where there are no constraints.

I’ll leave off with the usual list of resources that people have found useful. In particular, the ChibiKart instructable should be looked upon not as a example, but as a list of techniques and resources that (like what Ben’s done ) can be remixed into your own design.

Its worth noting that even a 2nd gen Li+ battery can put out enough energy to melt the tabs (!!!) even if its only rated for 1C.
I had this happen once, cost me an expen$ive pack even with an inline fuse the damage was done.

This is why model packs are typically rated at 20C, the tabs are 4* as thick and 2* as wide but they are porous to ensure a good seal between the outside world and the cells.
This also creates a weak point so strain relief is essential.